One drawback of the oil companies over the world is the production of water associated to oil. Premature breakthrough of formation or injection water and the rise in water production from then on results in accelerated decline of oil production, with increased pumping operation costs, treatment and discarding of huge volumes of water.
Water production reaches such amounts that it can be said that actually oil companies are water companies, since for an oil production of 75 million barrels oil per day (BPD), 300-400 million barrels of water are produced. This means 5-6 barrels of water per barrel of oil, attaining 9 barrels of water per barrel of oil or even 50 barrels of water per barrel of oil.
Fluid properties required for the control of permeability and mobility of water flowing in a porous medium are the following: 1) be easily injected at a long distance from the treated well; 2) bear properties such that the fluid weakly interacts with the physical chemical conditions of the reservoir; 3) should contain colloids or polymer species that are able to be adsorbed on several kinds of reservoir rocks for the formation of layers of controlled thickness; such layers should be hydrated in the presence of water and dehydrated in the oil phase; 4) such layers should be kept stable for long periods under the physical chemical reservoir conditions.
As cited in the paper SPE 77963 “New Insights into Water Control—A Review of the State of the Art” by Gino di Lullo e Phil Rae, water production in an oil well brings a collection of problems such as scale in susceptible wells, induction of fines migration or sandface failure, increased corrosion in tubulars, and killing of wells by hydrostatic loading. Thus, in spite of the fact that water production is an unavoidable consequence of oil production, it is usually desirable to defer its onset, or its rise, for as long as possible.
Water cuts in oil producing wells increases as the oil fields become mature. The source of the water is either formation water (aquifers) or injected water used for reservoir pressure maintenance purposes. In addition, heterogeneities encountered in reservoir rocks can cause water channeling through higher permeability streaks/hairline fractures (natural or induced) and/or near wellbore water coning due to the proximity of the well with the water-bearing zone, high rock vertical permeability, or high pressure differentials between the well and the formation normally caused by a limited reservoir thickness or excessive pressure drawdowns.
No matter the context, the origin of the problem is nearly always associated to a higher mobility of water relative to that of oil. Normally the lower the API degree of oil, the higher the possibility of premature water production, since low gravity oils are typically much more viscous, and hence have lower mobility.
The control of water production has obtained a limited success through the use of several gel-forming systems.
Three main chemical-based treatments are known.                Permeability Blockers or gellants;        Disproportionate Permeability Reducers and/or Selective Permeability Blockers; and;        Relative Permeability Modifiers (RPM).Permeability Blockers:        
As the name indicates, these products block or plug the pore spaces so as to hinder the movement of fluid, normally by means of a controlled, delayed chemical reaction that allows deep injection of the material before it reacts to form a three dimensional gel. Well-known examples of these processes are those involving sodium silicate solutions, internally activated with urea esters or aminoplast resins.
Other options include resins, latex or polymer solutions that gel up in response to temperature, salinity or pH to form coagulated precipitates or three dimensional gels, such as those created by latex or phenolic resins. The best-known systems are based on polyacrylamide crosslinked with chromium and PVA crosslinked with glutaraldehyde.
These products are not selective and they plug pores containing oil and/or water, thus requiring other procedures to avoid plugging off the oil-producing zone.
Selective Permeability Blockers
These products also plug the pore spaces, restricting fluid movement. However, due to their partial solubility in oil, precipitation, swelling or viscosifying is more reduced in the presence of hydrocarbons than in a water environment. The net effect is a reduction of water relative permeability by a larger factor than that to oil. Preferred products for use with water-based fluids (WBM) are rosin wood derivatives that form a colloidal precipitate that agglutinates, forming a gelatinous mass in the presence of water. Products for oil-based fluids (OBM) include tetramethyl orthosilicate (TMOS) and ethyl silicate that react in the presence of water to form a rigid silica gel.
There are also new systems based on viscoelastic anionic surfactant (VAS), designed for water control purposes. Such systems produce extremely shear thinning gels in the presence of cations. Such gels can easily permeate porous and permeable rocks, which allows to pump and inject them in the reservoir at full viscosity and higher matrix rates (below fracturing pressure). Once in the formation pores, the viscosity of such gels could increase as high as 100 times, thereby restricting fluid movement. The chemistry of such systems is such that hydrocarbons break them on contact and they revert to the base brine viscosity. This frees up only the pores with residual hydrocarbon saturation, leaving them clear and strongly water-wet. Highly water-saturated pores keep plugged with a high viscosity gel.
Relative Permeability Modifiers (RPM)
These are water-soluble, hydrophilic polymer systems that, when hydrated, produce long polymer chains that, in the rock, will loosely occupy the pore spaces. Being strongly hydrophilic, they attract water and repel oil and as a net result they exert a drag force on water flow in the pores, with a minimal and sometimes positive effect on oil flow.
Useful polymers for this purpose include high molecular weight polyacrylamides and more recently, scleroglucans. However, temperature limitations, shear sensitivity and poor tolerance to calcium and magnesium ions undermine their effectiveness, as they are produced back faster than expected by formation fluids.
Charged radicals have been added to polyacrylamides (poly-DMDAAC) improving their shear sensitivity, temperature and salt tolerance as well as their adhesion to rock.
Ideally, RPM systems should be aqueous solutions or suspensions, the features of which such as concentration and physical chemical properties should be adjusted to the formation permeability, among other parameters. RPMs provide a resistance (drag) to water flows in the order of 2 to 100 times and a detrimental drag to oil ideally lower than 2. Environmental changes such as pH, salinity or drawdown pressure will affect the effectiveness and durability of the treatment. Thus, post job interventions will most probably destroy, partially or completely, their water controlling properties.
SPE Paper 8228 “In-Depth Permeability Control by Adsorption of Soft Size-Controlled Microgels” by G. Chauveteau et al reports that the injection of stable, pre-formed microgels as relative permeability modifiers to reduce the permeability to water minimizes the risk of well plugging or the absence of efficiency inherent to a technology based on in-situ gelling. Recent investigations showed that microgels formed by crosslinking a polymer solution under shear are soft, size-controlled, and quasi-insensitive to reservoir conditions, stable over long periods of time and can control in-depth permeability by adsorbing onto all kinds of rock surface. The results shown in said paper are aimed at knowing how to control the kinetics of crosslink formation by ionic strength and at determining the role of the interactions between microgels on their propagation in porous media. Experiments include gelling tests at different ionic strengths, measurements of viscoelastic properties of solutions, determination of both microgel density and microgel-microgel interaction parameter for different stabilization conditions, and the relation between the interaction parameter and the mode of adsorption of microgels. Partly attractive microgels adsorb by forming multilayers and thus promote drastic permeability barriers. Fully repulsive microgels adsorb as monolayer and propagate easily in porous media at long distances, depending only on the quantity of microgel injected. Thus, by controlling both gelling and stabilization processes, microgels can be produced to be either diversion agents or disproportionate permeability reducers to control water permeability at long distances from the wells.
SPE paper 64988 “New Size-Controlled Microgels for Oil Production”, by G. Chauveteau et al., reports that microgels formed by polymer crosslinking under shear flow are very promising for several applications in oil production. The proper polymer/crosslinker system and under the conditions needed to obtain the desired properties provides quasi-ideal products. Such products are expected to control water mobility at long distances from the wells to improve sweep efficiency and reduce selectively permeability to water for water production control. This paper reports experiments related to the theoretical understanding of the crosslinking process under shearing and tests the microgels in porous media. Several microgel positive properties are reported.
SPE paper 59317 “Controlling Gelation Time and Microgel Size for Water Shutoff”, by G. Chauveteau et al., describes experiments designed to assess and control both size and conformation of microgels formed under constant shear flow. The reported studies indicate that the crosslinking species may be dimers, tetramers and associations of tetramers according to pH and Zr concentration in presence of lactate. Microgels formed in diffusion regime are isotropic and their size is significantly reduced as shear rate increases, while when formed in correction regime they are anisotropic and their size decreases negligibly with shear rate. Since experimental data are in agreement with such model, it is possible to design the microgel preparation as a function of its role in the aimed application, either relative permeability modifiers for water shut-off or viscosity enhancers for polymers flooding.
US Application 2004/0229756 relates to methods and solutions for treating water and hydrocarbon-producing formations for reducing the permeability to water thereof. The proposed solutions comprise a reactive hydrophilic polymer, a hydrophobic compound capable of reacting with the polymer in situ, and a surfactant. Solutions are prepared and injected in the formation followed by shutting-in the formation in order to permit reaction between the polymer and the hydrophobic compound. The reaction product attaches to adsorption sites on surfaces within the porosity of the formation and reduces the water permeability thereof without substantially reducing the hydrocarbon permeability thereof. The hydrophilic reactive polymer is selected among the group consisting of polyethylene imine, polyvinyl amine, poly(vinylamine/vinyl alcohol), chitosan, polysyline and alkyl acrylate polymers.
US Application 2004/0171495 teaches a method of reducing the water permeability of a well bore during the drilling phase, comprising: providing a polymer comprising: (i) a monomer selected from the group consisting of alkyl acrylates, alkyl methacrylates, alkyl acrylamides, alkyl methacrylamides, alkyl dimethylammoniumethyl methacrylate halides, and alkyl dimethylammonium propylmethacrylamide halides, wherein the alkyl groups have from about 4 to about 22 carbon atoms; and (ii) a monomer selected from the group consisting of acrylamide, 2-acrylamido-2-methyl propane sulfonic acid, N,N-dimethylacrylamide, vinyl pyrrolidone, dimethylaminoethyl methacrylate, acrylic acid, dimethylaminopropylmethacrylamide, trimethylammoniumethyl methacrylate chloride, methacrylamide and hydroxyethyl acrylate; and placing the polymer down hole, thereby reducing the water permeability of the well bore.
U.S. Pat. No. 6,579,909 relates to a method for preparing microgels of controlled size. According to the method, a gelling composition comprising a polymer and a crosslinking additive intended for the polymer is injected into a porous and permeable medium, and a monodisperse solution of microgels of substantially constant size is recovered at the outlet. The invention also relates to an application of the method for reducing the permeability of porous and permeable formations of reservoir rock type.
U.S. Pat. No. 6,516,885 describes that water shut-off is provided in a hydrocarbon well by injecting a first chemical composition, effective as a relative permeability modifier polymer, into the hydrocarbon and a water zone of the well followed immediately by the injection of a second chemical composition, which forms a flow blocking polymer composition, into the zones and, after a shut-in period for reaction of the relative permeability modifier polymer, back producing the polymer compositions from their hydrocarbon zones to provide a path for the hydrocarbons from the zones while allowing the flow blocking polymer compositions to form the water shut-off in the water zones. The hydrocarbon to which the process is applied is a gas. The first composition comprises a crosslinkable cationic polyacrylamide and the second chemical composition comprises a crosslinkable polyacrylamide-chromium acetate system.
U.S. Pat. No. 4,172,066 describes a composition, which comprises discrete, spheroidal microgels of a water-swellable polymer such as crosslinked polyacrylamide, useful for reducing the permeability of porous structures. Said microgels in the dry state have diameters less than about 20 micrometers, preferably lower than 4 micra, and still more preferably, lower than 1 micra, said polymer being sufficiently cross-linked to enable the microgels to remain as discrete spheroidal particles having diameters in the range from about 0.5 to about 200 micrometers, preferably between 1 and 10 micrometers when said microgels are dispersed in an aqueous fluid medium. In the partially water-swollen state the microgels contain at least 30% by weight of crosslinked polymer and up to 70% by weight of water. Useful monomers are the water-soluble ethylenically unsaturated amides such as acrylamide, methacrylamide and fumaramide, N-(diethylaminomethyl)methacrylamide and quaternized derivatives thereof, e.g., N-(trimethylammoniummethyl)acrylamide chloride; acrylic acid, methacrylic acid, itaconic acid, fumaric acid and the like; ethylenically unsaturated quaternary ammonium compounds such as vinylbenzyltrimethylammonium chloride.
U.S. Pat. No. 6,474,413, of the Applicant and hereby completely incorporated as reference teaches a process for the selective and controlled reduction of water permeability in oil-bearing formations made up of sandstone or limestone, the process comprising the injection of a slug of aqueous polymer solutions having a polarity opposite to the polarity of the rock, followed by the injection of a spacer slug of alkaline halide and then a fresh slug of polymer aqueous solution, the polarity of which is opposite to the polarity of the first polymer slug, and then a slug of aqueous solution of trivalent metal crosslinking agent to effect the partial crosslinking of the polymer charges, the polymer layers being successively added until the injection pressure of the polymer aqueous solutions show that the desired Residual Resistance Factor RRF has been attained, and well production may be resumed.
According to such process, spacer slugs consisting of an alkaline salt aqueous solution, such as a NaCl or KCl solution, always separate the polymer and crosslinking slugs. When the formation is rich in limestone rocks, initially an anionic polymer is injected, successively followed by cationic polymer slugs or multivalent crosslinking agent and anionic polymer slugs, the last layer being always made up of crosslinking agent. In this way the hydrophilic film formed bears an anionic character. All the polymer slugs are alternated with spacer slugs of an alkaline salt aqueous solution such as KCl. The process is useful for subterranean formations having permeability values of up to 3 Darcy.
The process described in said US patent is relatively cumbersome, since several polymer layers should be placed into the formation in order to reach the film thickness required for attaining the desired reduction in water permeability. On the contrary, in the present invention, the combined anionic polymer and controlled-particle size microgels used as second layer to be injected into the formation (in case of a sandstone formation) leads to the drastic reduction in the number of layers required for obtaining the desired RRF. This renders the present process more efficient, of lower cost and quicker to apply in the treatment of high permeability formations and high productivity wells. Besides, in the cited US reference the layered film forms a structure in the plane (two dimensional), while the present invention is directed to a structure forming a volume (three dimensional), where the microgel particles undergo hydration and dehydration according to their water or oil environment, in a reversible process.
Still, as cited on column 7, line 50 of the said U.S. Pat. No. 6,474,413, the process involves punctual crosslinking without gel formation, while in the present invention there is adsorption of the anionic polymer and also of the anionic microgel to the cationic layer, resulting in stronger/increased rigidity of the layer.
As compared to state-of-the-art processes using microgel only, an additional advantage of the invention results from combining anionic polymer and microgel besides the initial cationic polymer layer (for a sandstone formation), which provides better polymer adsorption to said initial layer. The net result is a rise in process efficiency due to lower polymer solution amounts to attain the desired effect of lower RRFw (Residual Resistance Factor) values.
Thus, when compared to state-of-the-art techniques—use of microgel only or the technique taught in U.S. Pat. No. 6,474,413—the process of the present invention provides potentialized results related to the FRRN ratio, those results not being described nor suggested in the cited known techniques.
Thus, in spite of the state-of-the-art developments, the technique is still in need of a selective process aimed at high productivity and high permeability fields using high molecular weight, seawater-soluble polymers combined to crosslinked acrylamide-based polymeric microgels, having a controlled particle size distribution for the reduction of relative water permeability in subterranean high permeability oil-bearing formations, while the permeability to oil is negligibly affected, such process being described and claimed in the present application.